The first edition of Understanding Wood was published in
1980 and has since sold more than 130,000 copies. It is widely held
as a definitive reference work and the cornerstone of every
woodworker's library. Now, Bruce Hoadley's comprehensive guide to
wood technology has been revised and updated in this
20th-anniversary edition of a classic.

New information on composite materials, adhesives, and finishes
brings this book into the 21st century, while more than 300
photographs bring important visual information to life. This edition
covers the nature of wood and its properties, the basics of wood
technology, and the woodworker's raw materials. Understanding
Wood was written for woodworkers by a scientist with a love of
woodworking. It will be sought after by craftsmen and collectors
alike.

"The truly serious woodworker needs a knowledge of his
material and the best place to get it is in Bruce Hoadley's Understanding Wood."-- UPI, WIRE, Chicago Sun-Times

"Not one craftsman in 100 could know a quarter of the
information Hoadley has collected in this handsomely illustrated
volume."-- American Forests

Introduction:

Twenty years ago I wrote the foreword to a book titled
Understanding Wood. I never imagined it would become a first
edition and that at the end of the century I would be working on a
second edition. This is certainly a time for reflecting on those
interim years, looking back and asking, "What is the same, what has
changed?" and at the same time pondering the future. As I've tried
to think about those 20 years of Understanding Wood, it would
be impossible to recall in detail the many personal thoughts and
reader feedback comments that come to mind. Nevertheless, the
retrospective overview leaves me with at least a few general but
indelible impressions.

First, I am flattered and humbled that the first edition has been
so well received. Apparently it has found shelf space with
woodworkers of all sorts from carvers and turners to cabinetmakers
and builders, both the novice and the expert. Even those whose
primary concern is not woodworking or even wood itself, such as
architects, engineers, museum professionals, and forensic
scientists, have found something useful in it. It is, after all, the
wood itself, not what we each do with it, that is the mystique.

Another great impression is the wonderful people I have met who
share the fascination of wood, and the breathtaking accomplishment
of their work. I have long marveled at the achievement of some of
the European carvers and furniture builders of the 17th and 18th
century and once believed that such work would never see an equal.
But I now look with astonishment at the works displayed at trade
shows and association annual meetings, in the pages of Fine
Woodworking and current books, demonstrating an extraordinary
level of skill. Certainly advances in woodworking tools, equipment,
and materials have contributed, but the skills and perseverance of
modern craftsmen cannot be denied. I have always enjoyed woodworking
myself, but I have succeeded only to the "jack-of-all-trades" status
and can only dream of the work I see rolling off the benches of
others. I like to think that Understanding Wood may have
helped here and there. I am proud just to be in their company.

A third thought was ever present as I considered the second
edition: The properties of wood are still the same with the
corollary that the same things that caused problems 20 years ago are
still the main troublemakers today. Wood-moisture relationships are
still the woodworker's arch-enemy. I have therefore tried to
maintain a focus on moisture-related properties and problems.

What, then, is new and different in this second edition? First,
converting black and white illustrations to color wherever possible
should improve both understanding and enjoyment of the subject
matter. In the chapter on wood identification, for example, the
gallery of color macrographs should make the comparison to pieces of
wood much more meaningful. Most important is the camera magic of
Randy O'Rourke, whose color photography will please the reader from
cover to cover.

Although wood itself has not changed, the availability and forms
have. The second edition has new sections and commentary on modern
products, particularly composites that were just evolving as the
first edition was written. As predicted, engineered wood products
have become a routine fiber-based commodity. With care, woodworkers
and builders can appropriately incorporate these new materials into
their work with functional and aesthetic success as well as economy,
for we have now passed through the era where "solid wood" was the
only mark of integrity. The changing availability of wood, together
with the development of newer chain saws and portable mills, has
developed a new incentive for going directly to the tree for
material, opening a new approach to enjoy woodworking. "Shopping"
for wood has also taken on some new angles, and I have added some
discussion of the new markets for wood and wood products from the
old familiar lumber yard to the internet.

As the bottom line, there is one thing that I think will never
change -- the unlimited enjoyment and satisfaction that await those
of us addicted to wood.
-- R. Bruce Hoadley, April 2000

Preface to the First EditionThe properties and characteristic qualities of the timber
available are so numerous and important, and yet so little
understood generally, that I am induced by solicitations of many
friends to give, in these pages, information respecting them.

A handy-book on timber is, in the opinion of many, much
required. The botanical treatises which are accessible are too
strictly scientific in their form and treatment to interest the
general reader, and they lack that practical application of
knowledge to the wants of the shipwright and carpenter, which is one
of the aims of this book to give.

I wish I had written those words, for they summarize perfectly
the reasons why I have written this book. But they were, in fact,
written in London in the year 1875 by Thomas Laslett, timber
inspector to the Admiralty of the British Empire, in his book,
Timber and Timber Trees.

Like Laslett, I have written my book for woodworkers, but this is
not a book about woodworking. Rather, it is about wood itself,
surely mankind's first workable material, and an ever-present part
of our ever-changing world. We are not likely to run out of wood,
because unlike most other materials, we can always grow more. And
the more we learn about it, the more there is to know. A look at
what else Laslett wrote in his book will make this clear.

Laslett believed, for example, that sap collected between the
bark and the wood eventually congealed to form a new growth ring. He
could not know about cellular reproduction and the additive
formation of new wood cell by cell. Laslett deduced that trees grew
taller because the bark squeezed the sap, forcing it upward. Today,
with a microscope, we can see cells building sideways in the cambium
layer, and twigs growing longer by cellular division. Lest we be
tempted to smugness, however, we should imagine how primitive our
scientific knowledge might look a hundred years from now.

The main reason I have attempted this book is the realization
that a wealth of knowledge about wood has been accumulated by
scientists, but almost none of it has been translated and
interpreted for the individual craftsman. Working from scientific
principles, technology is routinely developed by commercial and
academic agencies but mainly shared among themselves in highly
technical textbooks and obscure journals. The technology of
industrial-scale woodworking has been well developed and widely
published. But the same scientific principles have yet to be applied
to the small-scale woodworking shop, whether that of the serious
amateur, the independent cabinetmaker, or the artist/craftsman. For
example, volumes have been written on how to dry lumber in carload
quantities, yet it is almost impossible to find guidance when drying
the boards cut from a single tree, or when drying a single board.

Much of what has been written about the craft of woodworking is
reduced to sets of instructions of directions relative to tools and
procedures, without any supporting information about the material
itself. These how-to books commonly assume that all conditions
relative to the wood are under control, or else they dismiss the
point with an airy instruction such as, "Get some suitable hardwood
of appropriate quality and dryness." Yet, for a person to pursue a
craft with success, knowledge and understanding of the material must
develop along with manual skill.

I also feel compelled to attack the mountains of misinformation
available and commonly accepted by woodworkers. Most of it comes
innocently from the misinterpretation of observations. For example,
one book on sculpture states that bright light makes wood check, a
conclusion reached when cracks appeared in wood brought up from a
dark (and probably damp) cellar into daylight (and drier air).
Another says that wood cups because its annual rings try to
straighten out as it dries -- a correct observation of the direction
of cupping, but pure guesswork as to its cause. Tradition carries
along such misleading terms as "dry rot" and such misconceptions as
"wood has to breathe." Dry wood will not decay, nor does wood
breathe in the animal sense. Wood doesn't eat either, and it doesn't
require feeding with furniture polish.

My strategy has been to begin with the tree, to examine the wood
as the cellular product of the tree's growth. I have given special
attention to wood-moisture relationships and dimensional change
before going on to physical properties, strength in particular. Then
I have tried to analyze such everyday woodworking operations as
machining, bending, joining, and finishing in terms of the wood's
physical and biological nature. It is my hope that the examples I
have given can serve as models for readers in analyzing problems
that arise in their own woodworking endeavors.

I also hope that this book will encourage craftsmen to delve
further into the literature of wood science and technology, and to
help them do so I have included an annotated list of references.
Frequently, what makes technical literature opaque to the layman is
its terminology. Therefore, against the wishes of some who would
have me avoid scientific terms and "say it in simple words," I have
tried to present and explain the standard terminology throughout the
book, and I have included a detailed glossary. The serious
woodworker will find that it is important to know that rake angle
and hook angle have nothing to do with gardening or fishing,
and that terminal parenchyma is not a horrible disease.

I suppose every author wonders when a book actually began -- for
me, it goes back more than 50 years. I grew up in the Connecticut
countryside where the surrounding woodlands were both playground and
the source of material for "making stuff." The cellar of our house
had an old workbench and chests of grandfather's tools, worn from
years of use but begging for the chance once again to work miracles
in wood. My earliest memories include climbing a wobbly stool to get
on the workbench to turn on the light, the screech of the huge
square-threaded screw of the vise, and lifting the heavy lid of the
toolchest to stare at the mysteries within. I recall more trouble
than triumph from my early years of woodworking experiments. I
remember nails that bent over when driven into oak, saws that bound
up tight in green wood, screws twisted off when driven without pilot
holes, and planed surfaces ridged by nicks in the iron. But I still
"made stuff" -- my frustrations were nothing compared to my
fascination with wood.

I'll never forget the first time I saw a chain saw in operation.
It was at a late-summer farm-equipment demonstration, when I was in
my early teens. The farmers and loggers all watched in amazement as
the saw bar melted through a 12-in. oak log in a matter of seconds,
effortlessly taking slice after slice. It was quite a machine.

But I was not watching the raucous machine. I stood transfixed by
those marvelous disks of wood, a dozen or more, that lay in the
grass and sawdust. The demonstration over, the entire crowd followed
the saw operator back to his table to learn more about the machine.

Except me. I was excitedly stacking up as many of the wooden
disks as I could carry. They were red oak, creamy sapwood and medium
brown heartwood, just tinged with peach. As I staggered through the
goldenrod toward my father's car, they were unbelievably heavy. I
still recall the vivid pattern of the rings, their pie-crust of
bark, the cool dampness of the top disks under my chin, the pungent
odor of the wood. The aching in my arms was a tiny price to pay for
the unending array of things I would make from such a magnificent
product of nature: lamp bases, clock faces, desk sets, picture
frames. I could not believe that woodworkers had not already put
this beautiful natural log to better use.

My next recollection is of having the disks safely home and
proudly lined up along the wall shelf in our cellar workshop. I
probably realized that some sort of drying would be necessary, but
that could wait. I was content just to admire my treasure.

I am sure you already know the sad ending. By the following
morning, the brilliant end grain had faded to a lifeless sandy
color. In the days that followed, my castle of hope crumbled as the
first few hairline cracks in the sapwood grew and reached toward the
pith. Soon each disk had a gaping radial crack. In final mockery,
even the bark fell off.

But why?

It is easy for me to believe that this one incident was the
turning point that eventually led me to pursue my career as a wood
technologist, for ever since I have wanted to know why the wood does
what it does. I have been lucky enough to make some progress in my
quest, and I have been able to share in the classroom some of what I
have learned. Now I hope my book helps craftsmen understand wood,
too.
-- R. Bruce Hoadley, April 1980

4. Strength of Wood
Compression parallel to the grain
Compression perpendicular to the grain
Tension perpendicular to the grain
Tension parallel to the grain
Shear perpendicular to the grain
Shear parallel to the grain
Bending theory
The carrying capacity and stiffness of beams
Factors affecting strength properties
Compression failures and brashness
Structural grades